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Two 13-year-old boys have handed themselves in to police after a fire destroyed a seven-storey heritage building in central Sydney.

The building went up in flames on Thursday afternoon, requiring about 100 firefighters to extinguish the inferno.

Police believe other teens were involved in the incident and have asked them to come forward.

Fifteen people were sleeping rough in the building at the time of the fire and 13 of them have been accounted for.

However, at least 70 residents have been displaced from the surrounding buildings, with an exclusion zone expected to stay in place for seven days.

Police confirmed the teens who handed themselves in were assisting them with their inquiries.

Fire and Rescue New South Wales said the fire reached a "10th" alarm status on Thursday - the most severe level.

The building was heritage-listed and formerly home to the Henderson Hat factory. It had sat vacant for many years, but there were plans to redevelop it into a hotel.

After it caught fire, a thick column of smoke could be seen across Sydney. Video showed the top level of the building falling on to the nearby street.

Fire and Rescue NSW said they were able to contain the fire to prevent damage to nearby residential blocks.

The organisation also said investigations into the cause of the blaze had been taken over by NSW Police Arson Squad.

The building is located in inner-city Sydney, across the road from the city's central station.

Transport to and from the central area had to be stopped on Thursday as firefighters worked to extinguish the fire.